1. Field of the Invention
The present invention relates to a surface light-emitting device, such as a vertical cavity surface emitting laser (hereinafter also referred to as VCSEL), for use in, for example, optical communications, optical information processing, optical computing. The invention also relates to a compound semiconductor multi-layer mirror with layers including nitrogen for use in, for example, the surface light-emitting device. The invention also relates to a method for fabricating the surface light-emitting device, and an optical communication system using the surface light-emitting device.
2. Description of the Related Art
In recent years, a surface light-emitting device, especially VCSEL, has been actively studied because the device has a wide range of applications, including optical communications, optical information processing, and optical computing. The VCSEL is superior to an ordinary edge-emitting laser because (1) its structure is simple, (2) its operation current is small, and (3) the devices can be checked as a wafer unit on which a number of devices are formed.
On the other hand, the VCSEL also has several drawbacks. For example, the disadvantages listed below occur in a VCSEL having an oscillation wavelength in a 1.3 .mu.m-1.55 .mu.m band with an active layer of InGaAsP-series compound semiconductor when a distributed Bragg reflector (DBR) mirror is formed with a semiconductor multi-layer of InP/InGaAs(P). The disadvantages occur because a difference in refractive index between paired semiconductors (the multi-layer consists of a number of sets of paired semiconductors) of the mirror is small and thermal resistance of InGaAsP is several times larger than that of InP.
(1) Electrical resistance is high. Since the index difference between the paired semiconductors is small as described above, the number of layers of the multi-layer mirror must be increased to achieve high reflectance. Hence, the number of hetero-barriers increases and the thickness of the multi-layer mirror increases. Thus, when a current is injected through the DBR mirror, the electrical resistance thereof is very high. PA1 (2) Thermal resistance is high. Since the number of layers of InGaAsP having a large thermal resistance is increased, the total thermal resistance of the mirror abruptly increases. This lowers the efficiency of heat radiation by a heat sink. PA1 (1) High reflectance (in order to decrease the driving current), PA1 (2) Low electric resistance (in order to lower the driving voltage), PA1 (3) Low thermal resistance (in order to prevent a rise of temperature near the active layer), and PA1 (4) Readily builds the current constriction structure.
For these two reasons, a large amount of heat is generated, and the heat is likely to accumulate in VCSEL. Therefore, the temperature dependence of characteristics of injected-current vs. light-output in VCSEL (in particular, that in the 1.3 .mu.m-1.55 .mu.m band) is exceedingly degraded. That is, its characteristic temperature T.sub.0 is small and its maximum CW temperature is low.
To solve these problems, there is a known method of using a dielectric multi-layer mirror, which can attain a relatively-large index difference between paired layers, in place of the semiconductor multi-layer mirror. (See, for example, Journal of Japanese Academy of Electronics Information Communications, volume of J76-C-1, No. 10, pp. 367-374 (1993).) FIG. 1 illustrates a cross section of VCSEL fabricated by that method.
In FIG. 1, reference numeral 601 denotes an InP substrate. Reference numeral 602 denotes an n-type clad layer. Reference numeral 603 denotes an InGaAsP active layer. Reference numeral 604 denotes a p-type InP clad layer. Reference numeral 605 denotes a p-type InGaAsP cap layer. Reference numerals 606 and 607 denote dielectric DBR multi-layers of SiO.sub.2 /Si or the like, respectively. Reference numerals 608 and 609 denote annular electrodes, respectively. Reference numeral 611 denotes an etching stopper layer for use in forming a window (in which the dielectric DBR multi-layer 606 is formed) in the substrate 601.
In the VCSEL shown in FIG. 1, a desired reflectance can be obtained and its operation current can be reduced during pulsative operation. In the dielectric multi-layer mirror of FIG. 1, however, operation current tends to increase and light output is limited due to heat generation therein during continuous operation. Further, since the dielectric multi-layer is used instead of the semiconductor multi-layer, the multi-layer mirror can not be fabricated continuously along with other semiconductor layers. Thus, fabrication tends to be complicated and costly.
In a second known method to solve the above problems, a semiconductor multi-layer mirror of AlGaAs/GaAs, whose lattice constant is different from that of epitaxially-grown layers including an active layer, is fabricated separately from those epitaxially-grown layers. The separately-formed multi-layer mirror is then directly bonded to the epitaxially-grown layers by adhesive. (See, for example, Babic et al, "Double Fused 1.52 .mu.m Vertical-Cavity Lasers", Applied Physics Letters, 66(9), pp. 1030-1032 (1995).)
In this second known laser, a desired reflectance can be obtained (i.e., as seen from FIG. 7, a band gap difference corresponding to the index difference between AlGaAs/GaAs can be expanded, compared with InP/InGaAs(P), and the mirror of AlGaAs/GaAs does not function as an absorptive layer for light in the 1.3 .mu.m-1.55 .mu.m band (indicated by a dotted line W in FIG. 7)), and a current constriction structure can be readily formed by an oxidation process of the AlAs layer. Therefore, low-current operation can be initially executed. However, the laser has poor temperature characteristics because the active layer, which is vulnerable to heat, still exists. The laser is also costly and unreliable because three substrates (two for two sets of the semiconductor multi-layer mirrors and one for the epitaxially-grown layers), three semiconductor growth processes, and a bonding process are needed. Thus, the second known method is not yet a satisfactory method.
Furthermore, in recent years, III-V N obtained by adding nitrogen to a conventional III-V compound semiconductor has been reported. (See Kondow et al, "GaInNAs: A Novel Material for Long Wavelength Range Laser Diodes with Excellent High-Temperature Performance", Japan Journal of Applied Physics Letters, 35, pp. 1273-1275 (1996)). For example, a GaInNAs/GaAs quantum well active layer laid down over a GaAs substrate and a GaNAsP/GaNP quantum well active layer laid down over a Si substrate not only cover a range of 1.3 .mu.m-1.55 .mu.m (as is seen from FIG. 7, thick lines of GaInNAs and GaNAsP can be caused to cross the dotted line W at vertical lines of GaAs and Si which indicate semiconductors lattice-matched to GaAs and Si, respectively), but also offer a band-offset amount in a conduction band larger than that of an ordinary InGaAs/InGaAsP quantum well. Therefore, temperature characteristics of those active layers are expected to be drastically improved. However, an optimum structure of VCSEL using such new material has not yet been proposed.